There are several different conventional piezoelectric crystals, primarily in the form of quartz resonators, used to provide accurate frequency determination, stability, and precise timing. Increased use of the limited radio spectrum and the expanding mobile applications of electronic communication equipment, including sensor applications, serve to place greater demands on both the accuracy of frequency control and the magnitude of mechanical forces that such equipment must tolerate. Presently, almost all precision quartz resonators operate using bulk acoustic waves of the thickness shear variety. Among them the singly rotated AT cut is most commonly used, but the trend is toward greater use of doubly rotated cuts.
These thickness shear crystals or so-called thickness mode cuts, are in the form of thin circular discs. The mechanical contact area of the mounting clips are purposely kept small on the edge of the crystal plate to minimize the force-frequency effect. This effect involves the relationship between stresses due to the mounting supports applied to the crystal resonator and changes in resonant frequency. Unfortunately, the mounting geometry used in conventional crystal resonators has the capacity of translating any forces communicated between the quartz plate and mounting supports into high stresses within the relatively small mechanical contact area and attendant stress gradients in the vicinity thereof. Greatly increased sensitivity to damage of the quartz vibrator plate has been known to occur from this arrangement particularly where shock and acceleration are part of the environment.
A vertically of prior art approaches have been used to mitigate this persistent problem. For example, four points of fixation, instead of two, on the crystal resonator forming preselected mounting angles have been used. Another approach is to use double rotated cuts. However, all known arrangements with any degree of force-frequency insensitivity utilize circular plates peripherally mounted at points of limited area which inherently produces a concentration of stress at each mounting point.
It is therefore an object of the present invention to provide crystal resonators having mounting surfaces extending along a relatively large portion of the periphery of the crystal plate while preserving the force-frequency immunity of conventional crystal cuts.
A related object of this invention is to provide a crystal mounting having improved mechanical strength.
A further object of this invention is to provide a crystal whose lateral mounting surfaces are self-aligning and provide mounting surfaces which exhibit improved stability and resistance to mechanical forces and thermal shock.